Most airplanes are fitted with flaps for increasing the lift of the airplane while flying at relatively low speed such as while landing and while taking off. Depending on flying conditions, the flaps must be capable of being taken to different positions, generally a retracted position, and positions in which they are extended by 25%, 50%, 75%, and 100%, respectively.
Each wing of the airplane is thus provided with at least one flap. The flaps must be moved simultaneously and without twisting into their various positions, with any offset in position between the two flaps giving rise to an unbalance of the airplane that, if large, could lead to the airplane crashing.
The movement of flaps is thus generally provided by a single motor that is arranged in the fuselage of the aircraft and that rotates shafts extending inside the wings as far as the flaps. The shafts are coupled to the flaps in such a manner that rotating the shafts in one direction or the other causes the flaps to be extended or retracted.
Such devices are heavy, bulky, and installing them in an airplane is very constraining.